Add Azure storage account credentials for Workhorse and Rails

Heads up: Azure blob storage support is coming to GitLab, most likely in 13.4. These are the configuration changes needed to support this:

Workhorse configuration

The config.toml will look like this for Azure credentials:

[object_storage]
  provider = "AzureRM" 

[object_store.azurerm]
  azure_storage_account_name = "<YOUR ACCOUNT NAME>"
  azure_storage_access_key = "<YOUR ACCOUNT KEY>"

gitlab.yml

With consolidated object storage, the credentials are specified in azure_storage_account_name and azure_storage_access_key:

  object_store:
    enabled: true
    connection:
      provider: AzureRM
      azure_storage_account_name: '<YOUR ACCOUNT NAME>'
      azure_storage_access_key: '<YOUR ACCOUNT KEY>'

bucket is equivalent of Azure container name, so all that remains the same as before.

Related merge requests:

1. gitlab-org/gitlab-workhorse!555 (merged)
2. gitlab-org/gitlab!38882 (merged)